1. The Field of the Invention
The present invention relates to a method of testing optical materials by irradiating with high energy density radiation to determine long-term stability of their transmission properties, to the optical materials obtained with this testing method, and to uses of the optical materials obtained by this testing method.
2. The Related Art
Electronic computer components such as computer chips and other integrated circuits are manufactured by optical lithography. During their manufacture these circuit structures are imaged by a photomask on a support provided with a photo-lacquer, a so-called wafer, and the circuits and/or entire electronic devices including them are produced by irradiation. Since the requirements for computer performance are always increasing, increasingly smaller circuits are required. Because of that the respective circuit structures must be imaged ever more sharply, i.e. with greater resolution, which leads to the use of ever smaller wavelengths for irradiation of the photo-lacquer. However radiation with a shorter wavelength has a correspondingly higher energy.
It is also known that materials for optical elements absorb radiation passing through them so that the intensity of the radiation is generally less after passing through those materials than its initial value immediately prior to entering the materials. Moreover additional absorption and scattering effects occur at the surfaces though which the radiation passes, which similarly leads to a reduction of the transmission of the radiation. It is also known that the amount of absorption depends not only on the wavelength of the radiation, but also on the energy density or the fluence. When the path length of a light beam through the entire optical material of a lens system can be longer than a meter without more during irradiation of these smaller chip structures or circuits, absorption of the radiation passing through the lens system is a great problem. For these optical systems it is thus desired that the absorption is kept as small as possible, i.e. these systems and their elements should have a high permeability or transmission at least for the respective working wavelengths that are used in the system. It is also known that the absorption comprises material-specific (intrinsic) parts and so-called non-intrinsic parts, which result from inclusions, impurities and/or crystal defects. Intrinsic absorption is constant and depends on the nature of the material. It is independent of the quality of the material and thus does not decrease. The additional non-intrinsic absorption depends on the quality of the material, i.e. depends on the extent of the above-mentioned impurities, crystal defects, etc and thus can be avoided, at least theoretically. It leads to quality losses in the optical material and thus in the lens system.
Energy, which leads to heating of the material, is deposited in the optical material by intrinsic and non-intrinsic absorption. This sort of heating of the material has the disadvantage that optical properties, such as the index of refraction change, since the index of refraction depends not only on the wavelength of the light but also on the temperature of the optical material, which leads to changes in the imaging behavior in an optical component used for beam formation. Furthermore heating of an optical component leads to thermal expansion and thus to a change of the lens geometry. These phenomenon produce a change of the lens focal point and to some extent blur the projected image formed by a heated lens. In photolithography, as it is used for making computer chips and electronic circuits, this causes a decrease in quality and an increase in waste and thus is not desired.
Furthermore in many materials a portion of the absorbed radiation not only is converted into heat but also into fluorescence, which similarly is produced by impurities and crystal defects.
Attempts have thus already been made to determine the optical quality of these materials prior to their processing into optical elements. Thus WO 2004/027 395 describes a method for determining the properties of an optical material used for making optical elements, in which a radiation-induced absorption is measured in an optical material by irradiating it with an exciting radiation and measuring the total fluorescence comprising the intrinsic portion induced by this exciting radiation and the non-intrinsic portion. The non-intrinsic portion of the fluorescence is determined during and/or immediately after the irradiation.
In German Patent Document DE 103 35 457.3 A1 a method is described for quantitative determination of properties of the crystals used for optical elements at high energy densities, in which the radiation-dependent transmission at wavelengths in the ultraviolet (UV) is determined by radiation-induced fluorescence. In this method at least one induced fluorescence intensity maximum is determined by measuring nonlinear absorption processes at various fluences (H), determining the slope of the transmission curve from that determination, |dT/dH|, and the transmission from this slope. The so-called rapid damage process RDP may be established with this method.
In German Patent Document DE 100 50 349 A1 a method for determining the radiation stability of crystals is described. In this method the change of the absorption coefficient is measured before and after irradiation. In a first measurement the absorption spectrum A of a crystal, or of a piece of the crystal split off or cleaved from it, is measured over a predetermined wavelength range from λ1 to λ2 by means of a spectrophotometer. Then the crystal or cleaved piece of it is irradiated with an energetic radiation source for forming all theoretically possible color centers. After the irradiation the absorption spectrum B of the crystal or cleaved piece of it is measured in a second absorption measurement over the same wavelength range from λ1 to λ2. Subsequently the surface integral of the difference spectrum formed from the absorption spectra A and B over the range of wavelengths from λ1 to λ2 is formed and divided by the thickness D of the crystal. The absorption coefficient Δk induced by the working wavelength used in later applications is determined from this result.
European Patent Document EP 0 875 778 A1 states that the absorption of a CaF2 crystal is essentially caused by sodium impurities, which are typically in a range of about 0.1 ppm, in an image focusing optical system for a UV laser. According to that the other possible impurities, such as strontium, etc, contribute to the production of non-intrinsic absorption to an essentially small extent.
In European Patent Document EP 0 875 778 A1 a material to be tested is irradiated with an energetic ArF laser with a frequency of several hundred Hz for several seconds or minutes and the absorption spectrum prior to or after irradiation is determined. The irradiated energy per pulse amounts to 1 μJ to several Joule per pulse with a pulse duration of 10 to 20 ns. It was established that the absorption produced on irradiation of quartz glass and CaF2 with a laser does not correspond with that, which is measured by the weak light beam of a spectrophotometer. Thus it was found that the permeability or transmission of the material at the start of irradiation drops comparatively rapidly until after about 104 pulses and after that remains constant. Moreover it was expressly established that subsequently the transmission does not change further, so that the absorption is determined after about 104 to 105 pulses.
However all these methods only determine short-time, reversible radiation damage. This short-time reversible radiation damage is reversible by further irradiation or heat treatment, which means that the radiation damaged structures again relax. Up to now it was thought that the irreversible radiation damage that is known to occur in quartz glass, which causes a slow and irreversible increase in absorption during long-term use of this optical material over several years, did not occur in crystals. In the meantime however long-duration tests established that irreversible permanent damage also occurs in crystals after 108 to 109 laser pulses at energy densities of from 5 to 25 mJ/cm2 over a period of several weeks.
The current procedure for determining this permanent damage comprises determining the transmission T and/or the absorption A per input energy density or fluence H for the optical material measured and from that determining the slope of this curve |dT/dH| and/or |dA/dH|. The amount of absolute transmission or the initial absorption at an input energy density H=0 may then be ascertained from this curve by extrapolation to 0. This value normalized to the sample thickness is characterized as the initial absorption k0. Subsequently the optical material is irradiated with a higher energy density of about 1 Giga pulse with 10-12 mJ/cm2 and after this irradiation as described previously the initial absorption and/or absolute transmission is determined. The difference of the respective determined initial absorptions k0 (or absolute transmissions) before and after irradiation is a reliable measure for the long-term stability of the optical material. Optical materials with Δk0 values of >4×10−4 cm−1 have proven to be unusable.
Since the service life of this sort of lens system in steppers often amounts to ten years and more, a statement regarding the irreversible radiation damage that would occur over time is already required and unsuitable materials must be sorted out. Thus not only considerably costs for expensive manufacture of optical lens systems, but also illumination errors are avoided, whereby the yields during chip manufacture are increased. A simple determination of these long-term stabilities, which as much as possible is performable in a few hours, has currently not been possible and currently only occurs by the above-described pulsed laser shot method that takes several weeks.
The determination of long-term absorption increase in an endurance or stability test performed in a comparative short time has not been possible for practical reasons. Up to now no possible procedure has been found, with which a statement regarding the change of optical material properties over the entire service life can be obtained from experimental results obtained over a short time interval.